Which statement regarding protein kinases is true?
A. Cyclin binds to MPF to make Cdk, an active protein kinase.
B. Cyclin phosphorylates Cdk, turning it into MPF, an active protein kinase.
C. Cyclin phosphorylates MPF, forming Cdk, an active protein kinase.
D. Cyclin binds to Cdk to make MPF, an active protein kinase.